Mikvaot, Chapter Ten, Mishnah Seven
1) All foods combine together to make up the half of a half-loaf which makes the body unfit.
2) All liquids combine together to make up the quarter-log which makes the body unfit.
3) This is more of a stringency in the case of one who drinks unclean liquids than in the case of the mikveh, for in this case they have made all other liquids like water.
Section one: One who eats a impure food that is equivalent in volume to 1/4 of a loaf of bread (=volume of two eggs) is disqualified from eating terumah. If one eats several different types of impure food, they combine together to disqualify him (note that he is not impure, just disqualified).
Section two: Similarly, different types of liquid combine together to constitute the quarter-log of liquid that is required to disqualify a person from eating terumah.
Section three: The mishnah now notes, as it likes to do, that this makes the rules governing the drinking of unclean liquids more stringent than the rules governing a mikveh. For when it comes to a mikveh, only three logs of drawn water disqualify it. Other liquids do not. But when it comes disqualifying a person from eating terumah, where all liquids can join together.